1. Field of the Invention
The invention generally relates to a method and apparatus for image capture. Specifically, the invention relates to directing a laser video beam into parallel lines at a known location for use in a laser scanner.
2. Brief Summary of the Invention
There are two principal technologies used in image capture. The first relies on the use of a photosensitive sensor on which the image of a scene is optically projected. This technology is used for Imagers, Charged Coupled Devices known as CCD, and Active Pixel Sensors known as APS. The second uses a spot of concentrated light which sweeps the scene and whose radiation is sampled, quantified and spatially reordered to form an image. This technology is used for Laser Scanners.
Previous video frame capture was performed optically, for example, in a case of 2D CCD video image. Optical solutions suffer from poor depth of field. Contrarily, laser scanners have increased depth of field, but are incapable of video frame capture. To perform video frame capture with a laser it is necessary to record or device the time sequenced laser beam location. Previously, the laser beam location has been detected via the use of secondary circuits which monitor the reflected beam in a CCD matrix and/or window reflected light 2D CCD schemes. These solutions require additional components dedicated specifically to determining beam location. The present invention provides a solution that does not require additional components, thereby creating materials, manufacturing and energy consumption efficiencies.
The first technology is naturally directed towards frame grabbing, because the photosensitive pixels are spatially ordered and it is simple to extract an isometric image. Laser scanners read images in conjunction with oscillating mirros to automatically move the beam back and forth across the image.
It is more difficult to acquire an image with a sweeping device because guaranteeing with precision the position of the spot in the scene requires cumbersome and expensive hardware. Therefore the sweeping devices have been limited to applications of high added value such as display and 3D acquisition, or for the capture of one-dimensional images such as 1 D bar codes and stacked bar codes.
Laser scanners make it possible to obtain images of great depth, i.e. they can read images from far away, of field by the dissociation of the means of collection of light and spot production. Indeed, the spatial resolution of a laser scanner is related primarily to the size of the spot. The spot can be reduced to the limit of diffraction.
The spatial resolution of CCD devices depends not only on the size of the CCD pixels, but also on their sensitivity which requires a lens and aperture.
The last decade saw an emerging technology, Micro Electro Mechanical Systems (MEMS), which makes it possible to produce low cost deflectors with several degrees of freedom in a very small size, while oscillating at high speed and consuming little power. One can thus imagine a pen type 2D symbol reader with high depth of field, projecting a laser scanning of rectangular form delimiting precisely and with brightness the reading zone. It would be possible to change dynamically the sweep angles (zoom effect), making it possible to reduce the number of samples necessary.
Laser scanners lend themselves well to 3D frame grabbing by modulating the beam amplitude and by detecting the margin of dephasing of the reflected signal, making it possible to produce endoscope 3D.
The principal problem arising for acquiring a specially ordered image is the stability of the positioning function of the spot in the scene. There are several electro-mechanical technologies useful for stabilizing the positioning function. One can classify them into two families. The first family includes devices with high inertia and the second, those with low inertia. The high inertia devices use one or more rotating polygons which, by their accumulated kinetic energy, make it possible to make the angular velocity of the spot constant and insensitive to external disturbances. One can thus reach high speeds by the multiplication of the number of facets of the polygons and the use of synchronous motors or turbines with air cushion. These high inertia systems are cumbersome because of their mass and high energy consumption. Technologies with low inertia include oscillating systems actuated by electrostatic, piezoelectric, or magnetic forces. The weak inertia of these devices makes it possible to maintain them in resonance with little energy. One of their major defects until now were their relatively low (less than for 1000 Hz, angles of oscillation 40°) angular velocities.
A micro actuator MEMS consists of an oscillating assembly made out of a chip of silicon of a few tens of microns thickness and surface area approximately equal to that the spot. The absence of wearing parts (the bearings are replaced by two arms of silicon), and the reduction of the moment of inertia allow high frequencies of oscillation with little input energy. Moreover, the miniaturization of the deflector allows the packaging in a rarefied atmosphere which reduces damping due to air resistance and increases the amplitude of oscillation (less than 30 kHz, for angles of oscillation of 40°).
The present invention is a method of producing parallel, time sequenced laser beam location, intensity ordered pairs. This solves the laser video beam problem with a single component. Prior art methods rely on an external method of beam location such as direct linear CCD detection and window reflected light 2D CCD beam location. Prior art methods require additional components dedicated specifically to beam location. The present invention uses no additional components for beam location, other than the scanning mirror and information derived from the moving mirror.